I too use the classic freehand technique, I find that I have more control of how much tension I can apply and when it gets loose I can feel it and can make immediate adjustments on the fly.
I use my thumbnail to stay close to the edge of the wire wrapping and so far it seems to work perfect, that is right up until I hit the end and stop the drill. I have to remember that because it hurts like a son of a gun when the wire hits under the nail lol
Ahh, see I actually don't leave any distance between my thumb and the wires. It's easy to accomplish when you take spool out of the equation. I tie the outer wire to a doorknob, walk it back 20' and put very gentle, but constant tension on it till it gives a little - just enough so it falls slack when I let go. From there, I just let it rest on the floor while I work.
I pinch right between the tips of my finger and thumb. I seat it more under the wire than over. Index finger nail perpendicular to the core and thumbnail just about parallel. The wire never hooks under the thumbnail that way and it's just all-around easier, imo. It feels very bizarre. It's almost like your hand is automatically running along the bare core... ...feels far smother than you'd reckon it should. Like smoother than the bare wire itself. And then at the end, it's like shoop! Breaks-off and locks the outer to the core like magic.
You don't have to adjust anything - it's all automatic. The drill does all of the work for you. Just grab the fucker and go. With no tension on the outer wire, it doesn't tend to get wild on you so you don't really need to control or direct it once it falls into the groove. Your fingers just act like stoppers that keep it in that continuous groove - they just have to make it so the outer wire has nowhere else to go.
It's actually really easy to do once you figure out the angle and grip. Once it gets started, all you have to focus on is keeping that grip. You can make perfect claptons without looking. You can actually move your hands up, down, left, right, and back while you do it. Just don't tug it. The wire doesn't care as long as you keep the tension off.
Calluses are a real necessity for this - just to be able to keep the wire from slipping past your skin. It just won't work if you have bitch hands.
I know my technique is weird. I've never see anybody do it that way. It seems to work pretty well, though.